2008 SURI Intern

Year:      Junior
Major:    Physiology
School:  University of Arizona,
                Tucson, AZ

Mentor: Bob Goodman, Ph.D.
Dept:      Physiology & Pharmacology

Research Project: Discerning the Increased Sensitivity to Estrogen-negative Feedback in Sheep During the Anestrous Season: a Possible Connection Between Dopamine-containing Neurons and Kisspeptin Neurons Within the Caudal Arcuate Hypothalamus

The transition between pre-puberty and post-puberty in humans is an interesting topic of concern. No one really knows exactly how the transition is made. Apparently sheep and other seasonal breeders go through this cycle annually, which is why they are good subjects for research. Also, since this occurs in adults, changes in hormone levels that influence body growth instead of sexual characteristics can be ignored, which is much more difficult to do in humans. Sheep cycle between a breeding season and an anestrous (infertile) season. The most remarked difference between the seasons that produces the difference between fertility and non-fertility is the brain’s increased sensitivity to estrogen-negative feedback, which reduces the output of luteinizing hormone, an anterior pituitary hormone that controls ovarian function, via a reduction in the neuro-signaling hormone gonadotropin-releasing hormone.

Although the full process by which estrogen-negative feedback on GnRH is amplified during the anestrous season of sheep has not been discovered, pieces of the puzzle have been identified. Previous research has suggested dopamine-producing cells that are activated by estradiol-negative feedback only during anestrous produce the increased inhibitory signal that reduces the frequency of GnRH release, but they may not directly synapse with GnRH neurons. Other research has shown that kisspeptin neurons, when activated, excite GnRH neurons, and that they have direct neural contacts with GnRH neurons. This research project tests the hypothesis that the negative influences of the dopamine-producing cells directly inhibit kisspeptin neurons that are responsible for stimulating GnRH neurons. The caudal arcuate nucleus within the hypothalamus, which contains the kisspeptin neurons that synapse with the GnRH neurons, has been chosen for the site of testing. If this hypothesis is correct, a dopamine receptor (D2) antagonist injected into this area in an ovary-intact ewe should increase LH concentrations in blood samples because it blocks the inhibitory dopamine signal from getting to the kisspeptin neurons. A D2 receptor agonist injected in an ovariectomized ewe should inhibit LH concentrations because the ewe has no ovaries to produce estrogen, and therefore dopamine release onto the kisspeptin neurons should be minimized. To accomplish this, we will first implant chronic guide tubes aimed at the caudal arcuate. After a two-week recovery period, we will perform a dose-response using local injections of a D2 receptor antagonist with all ewes receiving all four doses in random order. Ewes will then be ovariectomized and a similar dose-response test will be performed with D2 receptor agonist. Finally, half the ewes will be given estrogen subcutaneously and tissue will be collected to determine if estradiol induces D2 receptor internalization. When D2 receptors are activated they become internalized within the cell, which is a useful marker for determining the activity of kisspeptin cells containing D2 receptors. Immunocytochemistry will be used to determine whether or not the predictions are correct by staining brain tissue samples from sheep for D2 receptors and kisspeptin neurons.

Click here to review the summary report of this project.